1. Field of the Invention
The present invention relates to an ATM switch for use in an ATM network and more particularly, to an ATM switch having a function of routing an IP packet.
2. Description of the Related Art
ATM router is the general name given to IP (Internet Protocol) routers having one or more ports for accommodating ATM circuits. The port herein explained means an interface which can connect physical circuits such as Ethernet, FDDI and ATM.
In case of a router which has only an Ethernet interface defined by an internet standard, IEEE 802.3, an incoming packet from a certain interface is generally output to another interface in accordance with a routing table present within the router.
In the ATM circuits, there is a concept of a virtual circuit (hereinafter, referred to as "VC") which can accommodate a plurality of circuits bundled in one physical circuit. Insome ATM routes, the same port receives a packet and transmits the same packet which is subjected to a routing process. That is, it is required for the ATM router that the routing process is managed by the VC rather than according to each port.
In a case where the IP packets are accommodated in the ATM circuits, an encapsulation method defined in RFC 1483 is employed. That is, when focusing attention to one VC, one IP packet is segmented into successive ATM cells before being transmitted.
Therefore, in the ATM router, the original IP packet is reassembled from these ATM cells and then subjected to a routing process before being segmented into the successive ATM cells again to be transmitted. That is, both the processes of reassembling the IP packet from the ATM cells and of segmenting the IP packet into the ATM cells are required.
A first disadvantage of the Prior Art is that two kinds of facilities, ATM routers and ATM switches, are required for constructing an ATM network.
A second disadvantage of the Prior Art is that the packet transmission capability of the network can not be improved unless the performance of the ATM router is improved, because the performance of the ATM router is a bottleneck for the packet transmission capability of the ATM network.